(1) Field of the Invention
The present invention relates to a liquid dispensing apparatus adapted for accurately metering and discharging a predetermined quantity of liquid including viscous fluid such as paste-like materials, adhesives and the like.
(2) Related Art Statement
Hitherto, such a liquid dispensing apparatus has been generally used in, for example, a die bonding machine to discharge a paste on a lead frame or a circuit substrate from a syringe in a constant quantity in order to bond a die on the frame or substrate with a base.
In this case, a quantitative discharge of the paste from the syringe is effected by supplying compressed air to the syringe which is filled with the paste. In a conventional paste dispensing apparatus, in order to control the air supply to the syringe, a discharging solenoid valve is arranged between the syringe and a compressed air supply and is controlled to open it for a predetermined duration.
However, the conventional paste dispensing apparatus as mentioned above is adapted for controlling only the discharging solenoid valve to open it for the predetermined duration and, as a result, the quantity of the paste discharged from the syringe is varied depending on a quantity of paste remaining in the syringe in spite of a constant compressed air supply duration.
If the paste is not transparent or the syringe is opaque or even if the paste is transparent, an operator cannot accurately confirm the remaining quantity of the paste by visual observation. In such a case, hitherto the remaining quantity of the paste in the syringe has been indirectly detected from a quantity of the paste squeezed out of the sides of the die at a die bonding process after the paste was discharged on a lead frame by the syringe. Then, the operator is required to reset the duration of the supplied compressed air or the set pressure of compressed air to an adequate value based on the detected remaining quantity of the paste.
Furthermore, if the operator cannot confirm the remaining quantity of the paste because the paste itself or the syringe is not transparent as mentioned above, and/or the operator is not watching the syringe when the die bonding machine is automated, the die bonding machine can be operated to such a state that the syringe contains little or no paste, so that the paste cannot be sufficiently discharged to be bonded at the subsequent die bonding process.
The conventional liquid dispensing apparatus of the prior art further has a disadvantage that the gravity causes the paste to drop little by little from the tip of the syringe during the stoppage of the apparatus.
In order to settle the aforementioned problems, there has been proposed a paste dispensing apparatus as disclosed in Japanese Patent Application Laid-open Publication No. 63-97,259.
This paste dispensing apparatus comprises a syringe containing a paste or the like, an air supply for supplying air under a positive pressure to the syringe, a discharging solenoid valve arranged between the syringe and the air supply and connected to the syringe by means of a first pipe line and to the air supply by means of a second pipe line for communicating the syringe with the air supply when the paste is discharged from the syringe, a first pressure sensor for measuring a pressure in the first pipe line, a second pressure sensor for measuring a pressure in the second pipe line, and a controller having inputs for signals of measured pressure from the first and second pressure sensors, outputs of voltage signals for actuating the discharging solenoid valve t communicate the first and second pipe lines with each other at the start of discharge and a control unit for controlling a duration of the output voltage signal by the time spent until the pressure measured by the first pressure sensor reaches a preset pressure after the first and second pipe lines are communicated to each other.
The aforementioned paste dispensing apparatus may further comprise an air suction for supplying air under a negative pressure to the first pipe line. The air suction is connected to said discharging solenoid valve via a third pipe line so as to communicate with the first pipe line while the first and second pipe lines do not communicate with each other.
The former apparatus mentioned above feeds a voltage signal from the controller, upon the discharge, to actuate the discharging solenoid valve, thereby supplying a positive air pressure from the air supply to the syringe to discharge the paste from the syringe. At this time, a build-up variation in the pressure measured by the first pressure sensor caused by the remaining quantity of paste in the syringe is estimated by the control unit. Correspondingly, a duration of an output signal from the control unit to the discharging solenoid valve, i.e., the discharging duration, is controlled so that the quantity of the paste discharged from the syringe is constant in spite of variation of the remaining quantity of the paste. Consequently, the paste can be discharged from the syringe with a constant quantity in spite of the remaining quantity of the paste in the syringe. Moreover, the latter paste dispensing apparatus can timely supply a negative pressure from the air suction to the syringe at the end of the discharge, thereby preventing the paste from dropping out of the syringe.
However, the former apparatus has disadvantages that pressure in the first and second pipe lines at the beginning of discharge is varied and directly influenced with variation of supply pressure of the air supply until the inner pressure in the syringe has reached the set pressure after a moment of communicating the syringe with the air supply, so that a building-up wave shape of a signal of the inner pressure in the syringe measured by the first pressure sensor is greatly disturbed and, as a result, the inner pressure cannot be measured by the first pressure sensor, and hence a discharge duration cannot be controlled by the control unit with high accuracy.
The latter apparatus also has disadvantages that time is required for reducing the inner pressure in the syringe to a predetermined negative pressure since a moment of communicating the syringe with the air suction after the paste has been discharged is directly influenced by the amount and pulsation of negative pressure supplied from the air suction, so that the weight of paste in the syringe cannot be quickly offset by the negative pressure. In other words, the dropping of the paste cannot be quickly prevented without a delay at the end of discharge of the paste.